1. Field of Invention
This invention relates generally to resonant cavity filters and more particularly to modular housings for filters having resonators as well as to the shape of those resonators.
2. Background Art
Conventional resonant cavity electromagnetic filters consist of an outer housing made of an electrically conductive material. One or more resonators is mounted inside the housing by use of a dielectric material. Electromagnetic energy is coupled through a first coupling in the housing to a first resonator, to any additional resonators in the housing and then out of the housing through a second coupling. The particular design, shape, materials and spacing of the housing and resonator will determine the signal frequencies passed through the filter, as well as the insertion loss or quality ("Q") of the filter. Many filter housings are designed with solid walls and are therefore suitable only for a preset number of resonators. If a particular application requires greater or fewer resonators than the preset number, an entirely new housing must be constructed to accommodate the desired number of resonators.
A type of resonator that may be used in filters is the split ring, consisting of a rectangular plate bent to form a hollow cylinder with a gap running from the top to the bottom of the cylinder at the ends of the bent plate. Thus while the sides of the cylinder or ring will be curved, the top and bottom will be flat and meet the cylinder sides at a right angle. Right angles or corners on the cylinder are undesirable for two reasons. First, the corners create discontinuities in the electric field around the resonator in the area of those corners. Second, resonators are generally made of a conductive material, but also may be coated with a superconducting material. Coating the corners of a resonator is difficult and may lead to non-uniform coating and additional discontinuities in the electric field around the resonator.
When the resonator contains superconducting material, additional problems are encountered because of the need to cool that material to cryogenic temperatures to achieve superconducting properties. The filter housing, resonator and dielectric material used to mount the resonator may all have differing coefficients of thermal expansion and are exposed to a very wide temperature range during cool down and warm up. Provision for expansion and contraction of the materials of such filters should therefore be made to ensure that all elements of the filter are properly situated when immersed in liquid nitrogen, or other super-cooling fluid or when attached to a cryogenic cooler. It is also important to ensure that the resonator is cooled sufficiently by the liquid nitrogen or other cryogenic cooling methods which will be located outside of the housing.